The present invention relates to magnetic particles for a magnetic toner, and a magnetic toner composed of such magnetic particles. More particularly, the present invention relates to particulate magnetic particles containing iron as the main ingredient, which display an excellent dispersibility when mixed with a low-molecular binder resin, especially, a low-molecular binder resin having a weight-average molecular weight of not more than 150,000, and a magnetic toner composed of such particles.
A development process using, as a developer, composite particles which are produced by mixing and dispersing magnetic particles such as magnetite particles with a resin without using a carrier, in other words, what is called a one component magnetic toner is well known and generally used in the electrostatic latent image development process.
With the recent improvement of the performances of copying machines such as the improvement in copying speed, picture quality, continuous operability and energy saving property, the improvement of the properties of a magnetic toner as a developer has been keenly demanded. For this purpose, magnetic particles which are well mixed with a binder resin are now in strong demand.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 65406/1970 as "Such a one component magnetic powder for a magnetic toner is generally required to have the following properties: ... VII) To be well mixed with a resin. The particle diameter of a toner is ordinarily not more than several 10 .mu.m, and the microscopic mixing degree of a toner is important to the properties of the toner ..."
Various improvements of a binder resin have also been investigated in order to improve the properties a magnetic toner. Aromatic vinyl resins such as styrene resins and vinyl toluene resins, acrylic resins such as acrylic acid resins and methacrylic resins, and copolymer resins of the monomers thereof are conventionally used as a binder resin used for a magnetic toner. These resins are high-molecular binder resins having a weight-average molecular weight of about 300,000.
However, the particle size of a magnetic toner has recently been increasingly reduced in consideration of a high picture quality. In order to obtain a magnetic toner having a small particle size, low-molecular resins having a weight-average molecular weight of not more than 150,000, which are easy to pulverize, have been put to practical use as a binder resin.
From the point of view of the copying-speed accelerating and the improvement of the energy saving property of a copying machine, it is eagerly demanded to use a low-molecular resin which enables a magnetic toner to be heat-fixed to paper at a low temperature and at a high speed, in other words, a resin having a low softening point. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 130547/1970 as "Although it is desirable that a heat-fixing developer has a low fixing temperature and an excellent preserving stability, if a resin having a low softening point is used in order to lower the fixing temperature, ..."
Various properties of magnetic particles used for a magnetic toner have also been examined in order to improve the properties of a magnetic toner. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 130547/1970, magnetic particles having an oil absorption of not more than 100 ml/100 g are proposed, and in Japanese Patent Application Laid-Open (KOKAI) No. 24950/1982, magnetic particles having a compressibility of 25 to 38% are proposed. In Japanese Patent Application Laid-Open (KOKAI) No. 182855/1989, magnetic particles having an apparent density of not less than 0.45 g/ml are proposed, and in Japanese Patent Application Laid-Open (KOKAI) No. 80/1990 (corresponds to U.S. Pat No. 5,066,558) , magnetic particles having a tap density of 1.2 to 2.5 g/cm.sup.3 and an oil absorption of 5 to 30 ml/100 g are proposed.
Although magnetic particles which display an excellent dispersibility when mixed with a low-molecular binder resin having a weight-average molecular weight of not more than 150,000 are now in the strongest demand, as described above, if known magnetic particles are mixed with a low-molecular binder resin having a weight-average molecular weight of not more than 150,000, it is impossible to obtain an adequate dispersibility. It is well known that when known magnetic particles are mixed with a high-molecular binder resin having a weight-average molecular weight of about 300,000, the smaller oil absorption the magnetic particles have, the higher the dispersibility thereof tends to be. On the other hand, when known magnetic particles are mixed with a low-molecular binder resin having a weight-average molecular weight of not more than 150,000, the smaller oil absorption the magnetic particles have, the lower the dispersibility thereof tends to be.
Accordingly, it is an object of the present invention to provide magnetic particles which display an excellent dispersibility even when mixed with a low-molecular binder resin having a weight-average molecular weight of not more than 150,000.
As a result of studies undertaken by the present inventors so as to achieve such purpose, it has been found that particulate magnetic particles containing iron as the main ingredient and having an oil absorption of not more than 24 ml/100 g and a degree of compaction of not less than 56 display an excellent dispersibility even when mixed with a low-molecular binder resin having a weight-average molecular weight of not more than 150,000. On the basis of this finding, the present invention has been achieved.